Commonly owned U.S. Pat. No. 3,624,304 shows a PAM telecommunication system with branched subscriber lines and with line switches that are closable in selected combinations to enable signal transmission between any two subscribers during periodically recurring sampling intervals, two consecutive sampling intervals being separated by a guard interval during which residual energy from previously intercommunicating subscriber lines is dissipated through an impedance matching the characteristic line impedance. For this purpose, a switch (which together with the associated matching impedance will be referred to hereinafter as a zero setter) is closed during the guard interval in order to minimize cross talk due to capacitively stored residual energy. Such cross talk may also be caused by energy electromagnetically stored in the line conductors; in contrast to the aforementioned capacitive energy transfer, which may be termed a transverse component of cross talk, the electromagnetic energy transfer--termed a longitudinal cross-talk component--is not affected by the zero-setting operation but can be minimized by a suitable conductor structure as described, for example, in commonly owned U.S. Pat. Nos. 3,878,485 and 3,973,227.
Transverse and longitudinal cross talk are two kinds of interference that are to be suppressed as much as possible in a well-functioning PAM telephone or other telecommunication network. Other parasitic phenomena impairing communication, such as attenuation and leakage, should also be closely controlled. Line segments, switches and possibly other circuit elements forming part of such a network should therefore be tested from time to time for the detection of faults liable to give rise to inadmissible cross talk or background noise.